The Ever Rising Seas

Since 1970 more than 90% of the extra heat trapped by the Earth as the climate warms has been absorbed by the oceans. The oceans respond more much more steadily than the terrestrial surface climate: they give us the real, underlying picture of the warming of the planet.

We have known for a long time that as the sea warms, sea levels rise – partly from melting ice sheets and glaciers, partly from thermal expansion, and partly for other reasons. People living in coastal regions know too this all too well they experience tidal flooding increasingly often.

Global sea level rise from 1970 until now has been at about 2.8 mm per year, but this graph hides the huge regional variation that exists.(noaa.gov)

We also know that sea levels are rising at surprisingly different rates around the globe. We’re starting to understand why.

In addition to thermal expansion (the ‘steric’ component), and meltwater from Greenland, Antarctica and land based glaciers, other contributors to sea level changes are hydrology (involving the water cycle of evaporation, precipitation and run-off), glacial isostatic adjustments (GIA), and a component called ocean bottom pressure (OBP)

Ocean bottom pressure refers to the weight of all the water on the bottom, ocean’s version of atmospheric pressure. It varies with tides, currents, winds, and water entering and leaving basins, and it changes over oceanic regions both seasonally and annually. It is difficult to incorporate into the picture, but the satellite gravimetry makes it possible, and it is an important driver.

The various contributing components to the rise of sea levels from 2002-2014. The lines are offset for clarity: the critical information lies in their slopes (pnas.org)

The overall view? From 2002 to 2014, ocean levels rose globally by an average of 2.74 mm/yr (the steepest, thicker grey line). Of that, 1.38 mm was the result of the thermal expansion of the warming water (the top orange line), and 1.37 mm the result of melting glaciers and ice sheets (the green, light blue and darker blue lines). Other contributors such as OBP and GIA appear on average to be variable and less important, while hydrology (the bottom green line), slopes downward, indicating a loss of water from the oceans to the land due to more rainfall, floods, groundwater loss, irrigation, and reservoirs, and so less runoff back into the oceans.

But those are global means, of little help for warning or advising coastal regions of what’s ahead.

This new study goes a lot further. It shows us not only how great is the regional variation in sea level rise, but also how much the various drivers vary in their contribution. The biggest surprise is that thermal expansion, the steric component, is about twice as great as previous estimates: sea levels are rising faster than we had thought.

The variation around the world is striking, to say the least. Look at the Philippines with mean sea level rise of a whopping 14.7 mm/yr, and Indonesia with 8.2 mm/yr. Thermal expansion of the warming water is the overwhelming contributor.

Components of sea level rise around Asian coasts. The boxed number on each pie chart is the mean sea level rise, mm/yr.(pnas.org)

The Northwest Atlantic (eg the east coast of the US) is the other region with the most rapidly rising sea level at 9.1 mm/yr) where glacial isostatic adjustments (GIA) still play a strong role: the land continues to subside from the elastic rebound following its release from glaciation. In contrast, along the west coast of the Americas and unlike most of the rest of the world, the sea level isn’t changing in any significant way.

Contributers to sea levels around the Americas. (pnas.org)

Obviously seasonal, annual and decadal variation in winds and currents play a critical role in modifying steric and OBP contributions, but the data appear to be robust and the general picture is likely to hold: the global average rise in seal level is accelerating, the steric component is greater than we expected, and global variation in rates of sea level rise are remarkable.

It is this last statement that is perhaps most important for the coastal countries of the world to absorb. Rising sea levels are already affecting some regions far more quickly than others.

What can we do about this? We cannot at this stage reverse the rise, and we probably won’t not be able to slow it for a very long time. warming sea water will continue to expand, polar ice sheets and glaciers will continue to melt.

What’s left is for us to adapt as intelligently as we can, pulling back from our current threatened shorelines.

Current hotel construction on Clearwater Beach, Tampa Bay, west coast of Florida (tampabay.com)